It seems that automounting is no longer working out-of-the-box in Gentoo.
Neither Gnome nor KDE would mount my USB stick in Gentoo with ~amd64 keyword.
The same USB stick was automounting in Gentoo about 1 year ago.

There is some output from udiskctl though when plugging in the USB stick:

No, I cannot mount it either, the error message is the same:
# mount /dev/sdf /mnt
mount: no medium found on /dev/sdf

I am quite lost, the stick was mounting before
and there is a filesystem which can be mounted.

The problem may be related to the particular
USB stick having a Realtek chip. I have tried
other USB sticks and they can be mounted.
However, the Hardware support for the
Realtek chip is enabled in the kernel,
and the same USB stick was mounting
with some older udev and before udisk
was introduced...

Yes, the problem was with the Hardware itself.
The USB stick is an adaptor for SD and micro-SD cards.

I was using two adaptors at the same time:
I was using a micro-SD card in a micro-SD-to-SD adaptor
which was then plugged into the SD card port of the USB stick.

Plugging in the micro-SD card directly into the
micro-SD port of the USB stick is working.
Thank you for all the feedback and support!

Now I just would like to learn about the new
handling of removable media in Linux.
Do you know of any good overview or Wiki web site for that?
It seems that the old mount points /mnt/xxxx
are no longer used and can be removed?
It seems that there is no "eject" functionality any more?
It seems that there is no notification in KDE or Gnome
about mounting the removable media in /run/media/<username> ?

For Gnome, useflags on gvfs matter, for KDE - IIRC it's solid.
The path, where it gets mounted, doesn't matter either way, as in both the backend receives the path in response, if mounting is successful.

I have solved this problem like this : I connected it to an android device (4.0.1+), it asked to format the device, I have waited and voila : the device is recognised now and fully working. I don't know why linux have trouble opening the device.